This invention relates in general to endless metal belt assemblies, and in particular, to endless metal belt assemblies with multiple belts in which the belt surfaces trap, circulate and carry a lubricant, thus minimizing friction between the belts.
Endless metal belt assemblies have many uses, including their use as a drive member for a continuously variable transmission (CVT). When used in this manner, an endless metal belt assembly must have certain properties and characteristics to operate efficiently.
The endless metal belt assemblies must be made of a material which is strong, exhibiting both a high fatigue strength which reduces the likelihood of failure from fatigue fracturing, and high compressive strength and tensile strength, which enable the belt assembly to withstand the demands imposed by the bending stresses inherent in the operation of the pulley system of the CVT. The belt material must be able to stretch without yielding, and be flexible. It must be durable with a high wear resistance, because replacement is costly and takes the machine out of use. The belt material must have high processability and be capable of being fashioned into a very thin belt which can be manufactured to a highly precise circumferential length. In the event of multiple belts forming the continuously-variable transmission belt assembly, this high precision of circumferential length for each successive belt is especially critical for the formation of equal gaps between pairs of adjacent belts. The metal belt assembly must have exacting tolerances with respect to the distance between belts, as well as minimal friction between belts. Thus, the adjacent opposing surfaces of the belts must be conducive to maintaining a lubricated state within the spaces between the belts. Each belt of a belt assembly must be capable of equal load sharing. The outer surface of the belt must have sufficient friction to transfer the load from the driving member pulley to the driven member pulley.
U.S. Pat. No. 3,604,283 to Van Doorne discloses a flexible endless member consisting of one or more layers of steel belts for use with a continuously-variable transmission, containing a driving mechanism which comprises a driving pulley with a V-shaped circumferential groove and a driven pulley with a V-shaped circumferential groove. The flexible endless member, which has chamfered (beveled) flanks, interconnects and spans the pulleys. The diameters of the pulleys can be automatically and steplessly varied with regard to each other in such a way that different transmission ratios can be obtained.
Endless metal belts used for belt drives can be formed by several methods. One manufacturing method disclosed in Metals Handbook, 9th edition, employs a "ring rolling method" wherein a metal, cylindrical tube is cut to a specified length and then an innermost belt is formed on the ring-rolling machine, making the ring wall thinner and the circumferential length longer. Subsequently, a number of additional belts wherein the diameter of each belt is slightly larger than that of the previously formed belt, may be similarly formed. The belts are then submitted to solution annealing in a vacuum furnace on a stainless steel cylinder, where the layered belts are rotated around two pulleys with tension in order to adjust the gap between the belts. After the dimensional correction, the layered belts are processed by precipitation-hardening (e.g., 490.degree. C. for 3 hours) and surface-nitriding. Finally, in order to improve lubrication ability between belts, surface profiling is performed. Such surface treatment may include grinding, rolling, knurling, peening, etching, abrading, conventional grinding, electrochemical grinding and selectively heating or selectively annealing specific portions of the belt.
U.S. Pat. No. 4,787,961 to Rush discloses a method of preparing multilayered endless metal belts, wherein tensile band sets are formed from a plurality of separate looped endless bands in a nested and superimposed relation. The patent states that the bands are free to move relative to each other, even though the spacing between the adjacent bands is relatively small. Such bands may be formed by an electroforming process.
When endless metal belt assemblies are used with continuously-variable transmissions, they experience frictional stress between belts, because in most applications the outside belt moves progressively faster than the next successive inside belt. Therefore, it is desirable to reduce the friction between the belts.